1. Field of the Invention
The present invention is related to an oscillating apparatus, and more particularly, to an adjustable oscillating frequency and oscillating apparatus with high power supply rejection.
2. Description of the Prior Art
In digital electronic devices, a clock generator is used for generating a clock signal, in which the clock signal is utilized as a reference clock for the digital circuit application. Normally, an external low-frequency clock source (e.g. a crystal oscillator) is utilized to generate a higher frequency clock inside the chip by a synthesizer. A self-resonated circuit within the chip generates reference clocks for some digital electronic devices. The former requires an off-chip oscillating device, and a synthesizer, which is implemented by a phase locked loop circuit; however the latter only requires a simple self-resonated structure, thus the latter is widely used in the driver of the small sized thin film transistor-liquid crystal display (TFT-LCD). Therefore, the chip area and power consumption of the latter are smaller than that of the former.
Ideally, when the self-resonated circuit oscillates, the resistor-capacitor (RC) value and the amplifier configuration within the self-resonated circuit will decide the range of the oscillating frequencies. However the variation of the external environment, such as supply voltage, manufacturing process, and temperature will change the oscillation condition of the self-resonated circuit and consequently cause a shift of the oscillating frequency from ideal conditions.
The oscillating period (T) of an oscillator can be approximated asT=(2CVSW)/I+(Xnon-linear term)=2RC+(Xnon-linear term),
wherein C is a charging/discharging capacitor, VSW is signal amplitude in the capacitor, and I is the average current. Xnon-linear term is a non-linear factor from voltage swing and current, and the active device also induces some non-linear effects.
Accordingly, the external environment is changed, the value of VSW, and I are also changed to cause the departure of the frequency of the oscillating signal from an ideal condition. Thus, the frequency derivation would be reduced by the external fixed resistance that generated the biasing current of the oscillator. The linear relationship between resistance and VSW/I in the oscillator portion would reflect the variation of oscillated frequency. Furthermore, the departure of oscillating frequency is apparently affected by the non-linear term that includes oscillator configurations and circuit limitations.